Magnetic Polarons Enable Exceptional Magnetocaloric Response

Abstract

Magnetocaloric materials are typically limited by a trade-off between magnetic entropy and field responsiveness. Here we show that magnetic polarons provide an intermediate regime that mitigates this constraint and enables an exceptional magnetocaloric response. Using EuB$_6$ as a model system, we combine thermodynamic and magnetic measurements to demonstrate that nanoscale ferromagnetic clusters emerging near the Curie temperature strongly enhance the field-induced entropy collapse. These clusters possess large effective moments that respond efficiently to applied fields while retaining substantial entropy due to their small size and dynamic fluctuations. As a result, EuB$_6$ exhibits a giant cryogenic magnetocaloric response, with both large isothermal entropy change and adiabatic temperature change in the technologically important 10-40 K range. Our results identify magnetic polarons as an underexplored route for optimizing magnetocaloric performance and establish an intermediate magnetic length scale as a design principle for high-performance cryogenic cooling materials.